Semiconductor light-emitting diodes (LEDs) are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness LEDs capable of operation across the visible spectrum are Group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Typically, III-nitride layers are epitaxially grown on sapphire, silicon carbide, or gallium nitride substrates. Sapphire substrates are often used, despite their poor structural and thermal match with III-nitride layers, because of sapphire""s wide availability, hexagonal symmetry, and ease of handling and pregrowth cleaning. See, for example, S. Strite and H. Morkoc, GaN, AIN, and InN: A review, J. Vac. Sci. Technol. B 10(4), Jul/Aug 1992, p. 1237.
To ensure LEDs with good performance, e.g., high brightness, high efficiency, or high reliability devices, the properties of layer interfaces must be carefully considered. Of particular interest are the layer interfaces below and within the active region. The quality of layer interfaces is controlled by the condition of the growth surface on which successive layers are deposited. Among conditions that lead to poor growth surface quality are substrate surface cleanliness, substrate surface misorientation, poor growth conditions, and impurities.
One method to achieve smooth GaN surface morphology is to grow a thick layer of GaN at high temperature (approximately 1100xc2x0 C.) and high group-V-to-group-III molar gas phase concentration ratios. GaN layers grown in such a manner have a high lateral-to-vertical growth rate ratio compared with GaN layers grown under standard growth conditions, allowing the GaN layers to overgrow rough surfaces and provide a smooth surface for the growth of subsequent device layers grown on the GaN layer. However, in order to achieve a smooth, planar surface, GaN layers grown in this manner must be thick and require a long growth time. Further, In-containing active regions in an LED or laser diode may require surface smoothness conditions that differ from conditions that can be provided by the above-described method.
In accordance with the invention, a graded smoothing region containing indium to prepare for active region growth is formed between the substrate and the active region of a III-nitride light emitting device. In one embodiment, the graded smoothing region has a graded composition. In another embodiment, the graded smoothing region has a graded dopant concentration. In some embodiments, the graded smoothing region is separated from the active region by a spacer layer having a constant composition and dopant concentration. The graded smoothing regions of the present invention may improve the surface characteristics of the layers grown over the graded smoothing region, particularly the active region.